Bug 794506 - Part 1: Move virtualenv code under mozbuild; r=ted

--HG--
rename : build/virtualenv/packages.txt => build/virtualenv_packages.txt
rename : build/virtualenv/populate_virtualenv.py => python/mozbuild/mozbuild/virtualenv.py
Bug 794506 - Part 2: Add virtualenv APIs; r=ted
Bug 917988 - Sphinx managed build system documentation; add mach build-docs; r=bsmedberg

--HG--
rename : python/mozbuild/dumbmake/README.rst => build/docs/mozbuild/dumbmake.rst
rename : python/mozbuild/mozbuild/frontend/README.rst => build/docs/mozbuild/frontend.rst
rename : python/mozbuild/README.rst => build/docs/mozbuild/index.rst
Backed out changeset 76f9cced8105 (bug 902715) for build bustage on at least Windows on a CLOSED TREE
Bug 917526 - Various duplicate symbol errors building tree with the 10.9 SDK. r=glandium
Bug 916845: Use size_t to describe length of source code in SpiderMonkey SourceHook lazy source hook. r=jorendorff
Bug 916845: Change sourceHook to a nice C++ object with a destructor. r=jorendorff
Bug 916845: Reindent ReadSourceFromFilename to match the surrounding style. r=bz
Bug 898349 - JIT support for getting and setting scalar properties and for optimizing away intermediate typed objects r=jandem
Bug 918808 part 1 - Don't call strlen for int32 values in NumberValueToStringBuffer. r=njn
Bug 918808 part 2 - Remove cruft from InflateStringToBuffer. r=luke
Bug 917242 - Symbolize ASan traces in automation. r=ted